1. Field of Invention
The invention is related to a thermal fixing device and an image forming apparatus having a thermal fixing device.
2. Description of Related Art
The device for forming an image by an electrophotographic method, such as a laser printer, has a thermal fixing device having a heat roller and a pressure roller for fixing toner transferred to a paper. Heat is applied to toner on a paper and the toner is fixed while the paper passes between the heat roller and the pressure roller.
The heat roller has a cylindrical shape and has a length corresponding to a whole width of a maximum size paper that can be used in a printer. A halogen lamp is arranged in the heat roller over a whole length in its axial direction. Almost the whole length of the heat roller in its axial direction is heated by heat generated from the halogen lamp. With this structure, the whole width of the maximum size paper can be heated.
In a normal printer, toner is fixed by one thermal fixing device for both a maximum size paper (for example, A4 size) and a minimum size paper (for example, B5 size) that can be used in a printer. In a thermal fixing device of the related art, the heat roller is always heated over a whole region in its axial direction. Therefore, when toner is fixed on a minimum size paper, the paper absorbs heat and the temperature is decreased in only a portion of the heat roller where the minimum size paper is contacted. However, temperature is not decreased at a portion of the heat roller where the paper is not contacted, i.e., the two ends of the heat roller.
On the other hand, because the temperature of a portion of the heat roller where a minimum size paper is contacted is controlled to be within a predetermined range (fixing temperature), the temperature of a portion of the heat roller where a minimum size paper is not contacted, that is the temperature of a surface of the two ends of the heat roller, becomes higher than the fixing temperature.
When toner is fixed on a maximum size paper after a fixing operation of a minimum size paper, the two end portions of the heat roller where the temperature is high are contacted to a maximum size paper. If the two ends portions of the heat roller are excessively heated, a hot offset is caused due to over-fixing of toner. The hot offset is caused when toner, that is melted too much, is adhered to a surface of the heat roller.
To prevent the above-described problem, after the fixing operation of a minimum size paper, the fixing operation is not executed until the surface temperature of a portion where the temperature is high becomes decreased to the fixing temperature. However, the efficiency of an image forming operation is extremely lowered.
In the invention, a fixing operation is executed rapidly with a stable fixing temperature at any time and without causing hot offset for printing mediums with different sizes.
In the thermal fixing device of the invention, a first region is provided in a heating element and a second region is continuously formed in the heating element. The heating element is heated by a first heat generating element and a second heat generating element.
A first heat region corresponding to the first region and a second heat region corresponding to the second region are provided in the first heat generating element. Thermal density A1 of the first heat region is higher than thermal density A2 of the second heat region. When the first heat generating element is turned on, a calorie generated by the first heat region is larger than a calorie generated by the second heat region.
A first heat region corresponding to the first region and a second heat region corresponding to the second region are provided in the second heat generating element. Thermal density B1 of the first heat region is lower than thermal density B2 of the second heat region. When the second heat generating element is turned on, a calorie generated by the first heat region is smaller than a calorie generated by the second heat region.
In a fixing operation of a small size printing medium, only the first heat generating element is turned on and the second heat generating element is off. The thermal density of the first heat region of the first heat generating element is higher than the thermal density of the second heat region and heat is not generated from the second heat generating element. Therefore, only the first region of the heating element is heated surely by the first heat generating element. Because a small size printing medium is transported while contacting the first region, the fixing operation of a small size printing medium can be executed precisely.
At this time, a small size printing medium does not contact the second region of the heating element, however, in the first heat generating element, the thermal density of second heat region is lower than the thermal density of the first heat region and heat is not generated from the second heat generating element. Therefore, in a fixing operation of a small size printing medium, the second region of the heating element is not heated excessively.
One heat generating element may have three or more heat regions. The heat element may be heated by three or more heat generating elements. In this case, the number of the heat generating element is N and there are provided a first heat generating element A, a second heat generating element B . . . a Nth heat generating element N. The first heat generating element has a first heat region, a second heat region . . . a nth heat region. Thermal density of each heat region is represented by A1, A2 . . . An. The second heat generating element has a first heat region, a second heat region . . . a nth heat region. Thermal density of each heat region is represented by B1, B2 . . . Bn. The Nth heat generating element has a first heat region, a second heat region . . . a nth heat region. Thermal density of each heat region is represented by N1, N2 . . . Nn.
The total (A1+B1 . . . +N1) thermal density of the first heat region of each heat generating element becomes smaller than the total (A2+B2 . . . +N2, A3+B3 . . . +N3 . . . An+Bn . . . +Nn) thermal density of each heat region other than the first heat region of each heat generating element. The thermal density of each heat region is determined so as to satisfy a relation of the following formula:
(A1+B1 . . . +N1) less than (A2+B2 . . . +N2)
(A1+B1 . . . +N1) less than (A3+B3 . . . +N3)
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(A1+B1 . . . +N1) less than (An+Bn . . . +Nn)